innervation to the RSR’s yoke muscle, the left inferior oblique (LIO). The LIO moves the eye upward and outwards, and it also responsible for extorting the eye. The extra innervation that was received by the LIO when it was behind the cover has now taken the eye too high and too far away from the nose to view the light. For this reason, to re-fixate, it is seen to move in a diagonal fashion, downwards and inwards, with perhaps a little intorsion evident.

5.27.5 Recording

Quantitative measures of the deviation angle (horizontal and vertical) are seldom needed in all directions of gaze. Indeed, the aim of these tests is to identify the likely source of the problem. Thus, the test result is normally recorded in a fashion which identifies the muscle(s) that is/are affected; e.g. ‘bilateral super rectus underaction’.

If quantitative measures are required, both vertical and horizontal prisms are needed to neutralise the movements on alternating cover test and during the Maddox rod or modified Thorington test in the various gaze positions.

5.27.6 Most common errors

1.Not keeping the viewing distance fixed in the various positions of gaze.

2.Failing to ensure occlusion occurs in peripheral gaze directions (cover test).

3.Switching the cover too quickly and not allowing the eyes time to take up position when prompted to fixate (cover test).

4.Not repeating the Maddox rod or modified Thorington test with the rod in front of the left eye.

5.26.7 Additional technique: double Maddox rod

By placing a Maddox rod in front of one eye the extent of an incycloor excyclo-rotation of the eye can be quantitatively evaluated. When the rod is placed horizontally the streak should, of course, appear to be vertically oriented. If the streak does

not appear vertical, this indicates the presence of cyclorotation and the rod orientation in the trial frame can be adjusted until the patient reports that the streak is vertical. The magnitude and direction of the rotation required to generate the impression of verticality is a measure of the nature and size of cyclorotation. The same procedure can be employed when a Maddox rod is placed before each eye. In this case a prism may be needed to dissociate the rods and the patient’s task is to assess whether or not they appear parallel. This is known as the double Maddox rod test and it is useful in bilateral conditions in which an underaction or overaction of the elevating or depressing extraocular muscles is present or suspected. The patient’s head should be in the primary position and the head should be held straight.

5.28 PARK’S 3-STEP TEST

The technique of assessing movements of the eyes as the head is tilted successively toward one shoulder and then toward the other was introduced by Hoffmann and Bielschowsky but has since come to be known as the Bielschowsky head tilt test (von Noorden 2002). The manner in which this test is normally used in the clinical setting is referred to as the Park’s 3-step test.

5.28.1 Advantages and disadvantages

This test can provide useful information to help determine the affected muscle in cases of known or suspected incomitancy. The test is useful in cases of paresis of any of the cyclovertical muscles, but the results are more dramatic when the oblique muscles are affected compared to when the vertical rectus muscles are involved. No additional equipment is required to perform this test. It offers the additional advantages in that the test is objective, allowing the practitioner to observe and measure the magnitude of any deviations without subjective responses from the patient, making it suitable for use in young children. However, the test result can be affected by a number of factors including the paresis of more than one muscle and mechanical restrictions by previous surgery to the extraocular muscles. Furthermore, interpretation of results may be more difficult than in the case of the Hess screen test.

5.28.2 Procedure

1.In order to carry out the Park’s 3-step test, the practitioner should attempt to answer the following three questions:

a)Which is the hyperdeviated eye in the primary position? The answer to this question may be obvious by simply viewing the patient or it may require the practitioner to carry out a cover/uncover test in the primary position (section 5.5).

b)Is the hyperdeviation greater in right or left gaze?

c)Is the hyperdeviation greater with head tilt to the right shoulder or to the left shoulder? This portion of the test is the Bielchowsky head tilt test.

5.28.3 Interpretation

Determine the muscle that the Park’s 3-step test suggests is paretic by matching the test result to the information provided in Table 5.7. If, for example, the right eye is the hyperdeviated eye in the primary position (Answer to Question 1), the deviation is greater on leftwards gaze (Answer to Question 2), and greater when the head is tilted to the right (Answer to Question 3), the muscle implicated is the Right Superior Oblique (RSO) (Table 5.7).

It is easier to recall the result patterns associated with the oblique muscles being affected. In the case of superior oblique muscles, the answers to the three questions will be right-left-right when the Right Superior Oblique is affected, and left-right-left when

the Left Superior Oblique is affected. In the case of the inferior oblique muscles, the result will be right- right-right in the case of the Left Inferior Oblique and left-left-left for the Right Inferior Oblique.

5.28.4 Most common error

It is not necessary to use prisms in order to complete the test. However, if prisms are used to measure/neutralise the deviation, the base must be held with its base parallel to the palpebral fissure when the head is in the tilted position, rather than parallel with the floor. This is to ensure that the prism has the same relation to the eye as in the primary position (von Noorden 2002).

5.29SACCADES

5.29.1Saccadic eye movements

Saccadic eye movements are used to quickly redirect our eyes so that an object of interest falls on the fovea. They are conjugate eye movements in that the eyes move by the same amount and they are the fastest of all eye movements with velocities as high as 700 degrees per second. Saccades serve several distinct functions:

■They occur at a frequency of about 20/minute and are used to continually scan the environment.

■Reflexive saccades occur in response to new visual, auditory or tactile cues.

■Voluntary saccades can be made to commands (‘look at the pen’) and to imagined or remembered target locations.

■They are particularly important during reading.

■They can also be voluntarily suppressed for the maintenance of steady foveal fixation.

■They form the resetting part of nystagmus and return the eyes to foveal fixation following a vestibular or optokinetic slowphase deviation.

Saccades originate in the left and right frontal eye fields (Broadmann’s area 8) of the frontal lobes.

5.29.2 Advantages and disadvantages

A simple assessment of saccadic eye movements can be made by direct observation of the patient’s eyes as they switch fixation from one target to another. No additional equipment is required and the test is very simple, quick and reasonably reliable (Maples & Ficklin 1988). A disadvantage of carrying out a dedicated test of saccades is that, if any abnormal pattern of saccadic eye movements does exist, it is likely to become apparent during other binocular vision testing. In addition, the information obtained is qualitative rather than quantitative. For this reason, the same test result may be interpreted in different ways by different practitioners.

4.Hold your right and left index fingers pointing upwards approximately 30° to 40° either side of the patient’s midline and at a distance of 40 to 50 cm from a point midway between the eyes.

5.Instruct the patient: ‘Please look at my right finger … Now at the other finger… and now the first finger again, etc.’ Repeat for about five cycles.

6.Grade the saccadic movements into one of four categories:

a)4 : Smooth and accurate.

b)3 : Slight undershoot.

c)2 : Gross undershooting, any overshooting or slight increased latency.

d)1 : Inability to do the task or greatly increased latency.

7.The assessment should be repeated monocularly if binocular problems are present or if the test result is 2 or 1 .

5.29.4 Recording

If normal saccadic eye movements are seen, record Saccades: 4 or 3 , or ‘smooth and accurate saccades’ or similar. If saccadic eye movements are abnormal (2 or 1 ) record the type of abnormality and indicate whether one eye is more at fault than the other.

5.29.3 Procedure

1.Seat the patient comfortably with their head erect and eyes in slightly downward gaze. Make sure the patient is wearing their near correction. Sit directly in front of the patient so that both eyes can be viewed simultaneously.

2.Keep the room lights on. Position additional lighting to illuminate the patient’s eyes or the target (whichever is necessary) without shadows.

3.Explain the measurement to the patient: ‘This test determines how well your eyes move to change their viewing position.’

5.29.5 Interpretation

All saccadic eye movements should be fast (completed in less than 1 second), conjugate and accurate, with no overshoots requiring secondary compensatory eye movements. A small undershoot with a compensatory eye movement is normal. Dysmetria denotes inaccurate saccadic eye movements and includes hypometria (undershooting) or hypermetria (overshooting). Abnormal saccadic eye movements could indicate ocular motor nerve paresis, internuclear ophthalmoplegia, myasthenia gravis, cerebellar disease, Alzheimer’s disease, Parkinson’s disease, gross visual field defects (saccades are used to keep the target within an intact part of the visual field) or could be due to systemic medications, particularly antidepressants. In cases of abnormal saccadic

eye movements, visual field assessment is warranted and referral may be necessary. It is worth remembering that disorders of saccadic, pursuit and fixational eye movements generally occur together.

5.29.6 Most common error

1.Misinterpreting the results due to lack of experience. For example, not realising that small undershoots are frequently seen.

5.30 BIBLIOGRAPHY AND

FURTHER READING

Carlson, N.B. and Kurtz, D. (2004) Clinical procedures for ocular examination, 3rd edn. New York: McGraw-Hill.

Ciuffreda, K.J., Levi, D.M. and Selenow, A. (1991)

Amblyopia: basic and clinical aspects. Boston: Butterworth-Heinemann.

Stidwell, D. (1998) Orthoptic assessment and management, 2nd edn. London: Blackwell Science.

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